As cast stainless steel suitable for machine parts and structure parts requiring high strength, SCS, SCH, etc. have been known conventionally. SCS is precipitation-hardened, martensitic, cast stainless steel containing Cu, Al, etc., which is provided with desired strength, hardness, toughness, corrosion resistance, wear resistance, etc. by turning a main phase in a matrix to martensite by a quenching or solution treatment (called “quenching treatment” summarily), and then forming precipitates of Cu, Al, etc. in the martensite matrix by a tempering or aging treatment (called “tempering treatment” summarily). Among them, SCS24 of JIS G5121 is a typical precipitation-hardened, martensitic, cast stainless steel containing Cu as a precipitation-hardening element, which is widely used for machine parts and structure parts for automobiles, vessels, construction machines, chemical plants, industrial machines, etc. However, the precipitation-hardened, martensitic, cast stainless steel tends to have poor cuttability (machinability), when provided with high hardness and strength.
As precipitation-hardened, martensitic, stainless steel having strength, hardness, toughness, corrosion resistance and wear resistance like SCS24, SUS630 is also known, but it has poor plastic workability (cold workability and warm workability) such as forging, rolling, extrusion, etc. and machinability in a tempered (aged) state, because of a structure having precipitates dispersed in a martensite matrix, and high hardness and strength. Accordingly, large plastic working or machining is conducted on the hardened SUS-type steel before tempering.
Proposed to improve the workability of the precipitation-hardened, SUS-type steel are, for instance, (a) to reduce C to 0.03-0.05% and N to 0.025-0.035% to lower hardness after a quenching treatment, thereby improving workability, (b) to add a small amount of S or Se to precipitate sulfides or selenides, thereby improving machinability, and (c) to optimize a composition range or quenching conditions, or conduct an annealing treatment during rolling to lower hardness after a quenching treatment, thereby improving workability.
However, the above methods for SUS-type steel are not suitable for improving the machinability of SCS-type cast steel. Decrease in C and N as interstitial solid solution elements in a martensite matrix extremely reduces castability, though it lowers the hardness of martensite. Particularly in cast steel with a complicated or thin shape, too little C fails to provide good melt fluidity, resulting in melt flow defects such as cold shuts, misrun, etc. Also, only the addition of S or Se fails to improve machinability sufficiently. Although any of the above methods improves workability after quenching, it does not consider workability after tempering.
Precipitation-hardened, martensitic stainless steel cast in a shape close to a final product (near-net shape) is usually roughly worked after quenching, tempered to have high hardness, strength, wear resistance, etc., and then finish-worked to remove scale and strain generated by the tempering treatment and to have desired surface roughness and dimensional accuracy. Thus important for the precipitation-hardened, martensitic, cast stainless steel are not only machinability after quenching but also machinability after tempering.
JP 2004-332020 A proposes SUS-type, precipitation-hardened, martensitic, stainless steel having a composition comprising by mass 0.005-0.030% of C, 0.1-0.5% of Si, 0.1-0.7% of Mn, 5-6% of Ni, 15-17% of Cr, 0.5-1.5% of Mo, 2-5% of Cu, 0.10-0.40% of Nb, and 0.005-0.030% of N, the rest being Fe and inevitable impurities, which is subjected to (1) quenching at a relatively low temperature to have a low-strain martensitic structure in which small amounts of C and N are dissolved, (2) a first aging treatment comprising keeping the cast stainless steel at a high temperature of 700-800° C. for 15 minutes to 20 hours, and then cooling it to room temperature, thereby making Cu, a precipitation-hardening element, coarser to lose hardenability, and then (3) a second aging treatment comprising keeping the cast stainless steel at a temperature of 600-680° C., at which the maximum amount of reverse-transformed austenite is formed from a martensite phase, for 15 minutes to 20 hours, and then cooling it to room temperature, thereby precipitating 30% or more by volume of low-hardness, reverse-transformed austenite for its interconnection to improve machinability after tempering. This precipitation-hardened, martensitic, stainless steel is provided with reduced hardness after a solution treatment with the amounts of C and N reduced, and has excellent machinability by the structure control steps (1)-(3).
However, this precipitation-hardened, martensitic, stainless steel has poor castability, because the C content is 0.03% or less by mass to reduce hardness. Also, because as much reverse-transformed austenite as 30% or more by volume is precipitated to improve machinability, the machinability is extremely deteriorated by deformation-induced, martensitic transformation, when used for cutting. In addition, because the first and second aging treatments (corresponding to tempering treatment) are conducted at higher temperatures than usual after a solution treatment (corresponding to quenching treatment), it needs a larger number of heat treatments with larger energy consumption, so that the steel is likely to have heat treatment strain that cannot be removed easily, resulting in a higher production cost.
There are thus various proposals to improve workability in a quenched state in the SUS-type, precipitation-hardened, martensitic, stainless steel, and a proposal to improve machinability in a tempered state (JP 2004-332020 A). However, with respect to the SCS-type, precipitation-hardened, martensitic, cast stainless steel, there is no proposal to improve machinability in a tempered state.